The invention relates to transporting telephony signaling, such as stimulus messages for a digital telephone, over a data network.
Private telephony exchange systems may include private branch exchange (PBX) systems, key telephone systems, and Centrex systems (in which a central office exchange provides PBX-like switching for a special line group). Typically, a private telephone exchange system is characterized by a simplified number plan that identifies extensions by five or less digit numbers, depending upon the size of the exchange system. This is in contrast to a minimum of seven digits (or even ten) typically employed for a directory number serviced by a public central office exchange. Private telephone exchange systems may also offer other services, such as voice mail, intercom, message waiting indication, and other features.
A private telephone exchange system includes interface circuits (e.g., line cards of a PBX system) to communicate with various telephony devices, which may include digital telephones. Native stimulus messages may be exchanged between the telephones (which may also be referred to as “stimulus telephones”) and the private telephone exchange system for performing calls. The types of stimulus messages that are exchanged between a stimulus telephone and the private telephone system includes the stimulus telephone reporting key press and hook state events to the switch system, and the switch system communicating commands to the stimulus telephone to cause activation of the telephone's ringer, control of the telephone's display, and activation and connection of the telephone's handset to the audio path. The stimulus telephones are connected to the exchange system over a number of corresponding lines. A feature offered by the private telephone exchange system is that the number of lines coupling the telephones may exceed the number of central office lines that the exchange system is coupled to. Thus, a first number of central office lines may be shared by a second number of telephones through the telephone exchange system, with the second number typically much larger than the first number.
Stimulus telephones rely on the telephone exchange system for performing various functions. As such, stimulus telephones are not provided with much intelligence. However, advantages of stimulus telephones are that they are relatively cheap to manufacture and are not complex.
One type of interface between a stimulus telephone and a private telephone exchange system employs a time compression multiplex (TCM) technique on a half-duplex transmission link, in which a burst mode or ping-pong approach is used. Typically, in the TCM link, the digital information signal to be transmitted is divided into discrete portions, with each portion time compressed to form a “burst” that occupies less than one half the time of the original portion. The transmitter at each terminal connected to the TCM link alternately transmits the burst onto the link, following which the associated receiver at each terminal can receive a corresponding burst from the other transmitter. On receipt, each burst is expanded to occupy its original time span. Externally, the system appears to be transmitting the two digital information streams continuously and simultaneously (full-duplex communication).
Other types of interfaces exist for carrying stimulus messages between a stimulus telephone and a private telephone exchange system. Such interfaces are typically proprietary and may differ depending on the manufacturer of the telephone exchange system and stimulus telephones.
One limitation of private telephone exchange systems is that they may be limited in geographical reach. Thus, typically, a private telephone exchange system is employed at a location in which telephone sets are relatively close to the telephone exchange system. As the size of a facility grows, additional private telephone exchange systems may be added to increase capacity and to provide service for different segments of the facility.
However, private telephone exchange systems usually do not reach remote sites (such as remote office location or a home office location) using conventional links between stimulus telephones and the private telephone exchange system. For such remote locations, different setups are typically provided. In the home office location, for example, the user may subscribe to a dedicated central office line for communicating over the public switched telephone network (PSTN). At a remote office location having a relatively low number of users, a separate exchange system such as a key telephone system may be employed to support those users. With either the dedicated central office line or separate telephone exchange system approach, however, users are not hooked into the one or more private telephone exchange systems at the main office location. This prevents convenient user access to various features offered by such private telephone exchange systems, such as voice mail, extension dialing using a reduced number of digits, and intercom features.
Another issue associated with private telephone exchange systems is that dedicated telephone lines are typically required to connect the stimulus telephones to the telephone exchange system. Such telephone lines are usually in addition to cables that are run for a local area network (LAN) to provide data services. A LAN is typically coupled to computers, servers, gateways, routers, and other devices to enable data communications over the LAN between the network elements. The presence of separate lines for telephony services and for data services may be associated with increased costs.
A need thus exists for an improved method and apparatus of coupling telephony devices such as stimulus telephones to a telephone exchange system.